GROUP-IV SOLAR CELL STRUCTURE USING GROUP-IV or III-V HETEROSTRUCTURES

ABSTRACT

Device structures, apparatuses, and methods are disclosed for photovoltaic cells that may be a single junction or multijunction solar cells, with at least a first layer comprising a group-IV semiconductor in which part of the cell comprises a second layer comprising a III-V semiconductor or group-IV semiconductor having a different composition than the group-IV semiconductor of the first layer, such that a heterostructure is formed between the first and second layers.

TECHNOLOGICAL FIELD

The present disclosure relates to photovoltaic cells and their methodsof manufacture and, more particularly, to improved single junction cellsor subcells in a multijunction photovoltaic (PV) cell, especially asolar cell, having multiple layers to form a hetero structure.

BACKGROUND

Advances in photovoltaic cells remain important for terrestrial andnon-terrestrial applications. In the non-terrestrial environment ofouter space, as well as terrestrial applications, photovoltaic cellsoffer a valuable means for providing power generation by converting theabundant resource of the sun's energy to electrical power.

Irrespective of the application, and as with any energy generationsystem, efforts continue to increase the output and/or efficiency of PVcells. With respect to output, multiple cells or layers having differingenergy bandgaps have been stacked so that each cell or layer can absorba different part of the wide energy distribution of photons found insunlight. Stacked arrangements have been provided in monolithicstructures on a single substrate, or on multiple substrates.

In the multiple cell device, known as multijunction solar cells,multijunction photovoltaic cells, or multijunction cells, semiconductormaterials are typically lattice-matched to form the solar cells withinthe multiple cell device, known as subcells within the multijunctionsolar cell. Each subcell typically contains at least one collecting p-n(or n-p) junction. A multijunction solar cell with 2 subcells istypically called a 2-junction cell; one with 3 subcells is called a3-junction cell, etc. so that cells with n subcells are calledn-junction cells, where n is an integer.

Additionally, the subcells within a multijunction cell are ofteninterconnected in series by tunnel junctions between subcells, that actas low resistance contacts that typically are not photoactive. Incontrast, the collecting junctions in each subcell typically arephotoactive. The term photoactive means that a given photoactive layer,structure, subcell, etc. within a solar cell contributes to the outputcurrent and/or voltage of the overall solar cell, in response to lightincident on the solar cell. As described earlier in the text, thenumbering of 2-junction (2J), 3-junction (3J), and, in general,n-junction (nJ) solar cells is determined by the number of subcells, orcollecting junctions, not including tunnel junctions.

The collecting junction of a photovoltaic solar cell or subcelltypically consists of a p-n junction between a layer of one doping type(either p-type or n-type), typically called an emitter layer, andanother layer of the opposite doping type, typically called a baselayer. The junction may also consist of a p-i-n junction in which anintrinsic semiconductor layer with little or no extrinsic dopantconcentration is placed between the emitter layer of one doping type andthe base layer of the opposite doping type. Typically, the emitter layeris considered to be the layer that is closer to the primary light sourcefor the solar cell than the base, and the base layer is considered to bethe layer that is farther from the primary light source than theemitter. Typically, the front surface of a solar cell or solar cellcomponent is considered to be the surface closer to the primary lightsource for the solar cell, and the back surface of a solar cell or solarcell component is considered to be the surface farther from the lightsource. However, there can be exceptions to this typical terminology,for instance when both back surface and front surface illumination areincident on the solar cell.

Both the collecting junctions and the tunnel junctions can be of thehomojunction or heterojuction type. When solar energy is absorbed by asubcell, minority carriers (i.e. electrons and holes) are generated inthe conduction and valence bands of the semiconductor materials adjacentthe collecting junction. A voltage is thereby created across thejunction and a large portion of the photogenerated current can becollected at a finite voltage to generate electrical power. As thephotons in the solar spectrum pass to the next junction that typicallyhas been optimized for a lower energy range, additional photons in thislower energy range can be converted into a useful current. With agreater number of junctions, greater conversion efficiency and increasedoutput voltage and electrical power can be obtained.

With multijunction cells, efficiency is limited by the requirement oflow resistance interfaces between the individual cells to enable thegenerated current to flow from one cell to the next. In a monolithicstructure, tunnel junctions have been used as described earlier in thetext to minimize the blockage of current flow. In a multiple waferstructure, front and back metalization grids with low coverage fractionand transparent conductors have been used for low resistanceconnectivity.

Another limitation to the multiple cell PV device, or multijunctioncell, is that current output in each subcell must be the same foroptimum efficiency in the series-connected configuration. In addition,there has been a practical limit on the number of subcells (alsoreferred to as collecting junctions or subcell junctions) employed,since the benefit of each successive subcell becomes smaller as thenumber of subcells increases, and each subcell has certain parasiticlosses associated with it in practice, tending to counteract the greaterefficiency that comes from dividing the incident spectrum into smallerenergy ranges with a greater number of subcells.

The concern over efficiency in PV cells has created interest inoptimizing 1-junction solar cells such as silicon (Si) cells and galliumarsenide (GaAs) cells, and in developing higher-efficiency multijunctioncells such as conventional 3-junction GaInP/GaInAs/ Ge solar cells,which employ a gallium indium phosphide (GaInP) top subcell, also calledcell 1 or C1, a gallium indium arsenide (GaInAs) subcell in the cell 2(C2) position, and a germanium (Ge) subcell formed by a Ge wafer growthsubstrate in the cell 3 (C3) position. In this 3-junction solar cell,cell 3 is also the bottom subcell. Thus the subcells are numbered in theorder in which the incident light passes through the multijunction cell.The material name associated with a solar cell or subcell is typicallythe material that is the dominant photoabsorber in the cell, which istypically the base of the solar cell.

The structures described above have relatively high current densities,which can present problems for current matching subcells that are poorcurrent producers. The comparatively high current densities and lowvoltages of these cells with 1 to 3 subcells result in greater relativepower loss due to series resistance. Further, subcell base thicknessescan be comparatively large, and some subcells have little excessphotogenerated current density, both of which impair radiationresistance. Limited excess photogenerated current density in low bandgapsubcells can also impair the fill factor of the overall multijunctionsolar cell that they are in.

BRIEF SUMMARY

The present invention is directed to a photovoltaic cell comprising twoor more group-IV solar cells which are subcells within a multijunctioncell, where at least one group-IV subcell is an upper subcell positionedcloser to the primary light source than at least one other group-IVsubcell which is a lower subcell, and where the at least one uppergroup-IV subcell has a higher direct and/or indirect energy bandgap thanthe at least one lower group-IV subcell. (See FIG. 12).

According to the disclosure, the photovoltaic cell may further comprisean upper subcell comprising at least one group-IV layer in which themajority of photogeneration for that upper subcell takes place, with adirect and/or indirect bandgap higher than the group-IV layer in a lowersubcell in which the majority of photogeneration for that lower subcelltakes place, where the compositions for the group-IV layers in the upperand lower subcells may be selected from a group including: Ge, Si, SiGe,CGe, GeSn, SiGeSn, and CSiGeSn.

According to the disclosure, the photovoltaic cell may further comprisean upper subcell comprising a Si, CGe, SiGe, SiGeSn or CSiGeSn layer inwhich the majority of photogeneration for that upper subcell takesplace, and a Ge, SiGe, CSn, SiSn, GeSn, SiGeSn, or CSiGeSn layer in thelower subcell for which the majority of photogeneration for that lowersubcell takes place.

According to the disclosure, the photovoltaic cell may further comprisean upper group-IV subcell 1 comprising at least one group-IV layer inwhich the majority of photogeneration for that subcell 1 takes place,with a direct and/or indirect bandgap higher than the group-IV layer ina group-IV subcell 2 in which the majority of photogeneration for thatgroup-IV subcell 2 takes place, where in turn the group-IV subcell 2 hasa direct and/or indirect bandgap higher than the group-IV layer in agroup-IV subcell 3 in which the majority of photogeneration for thatgroup-IV subcell 3 takes place, continuing in this way for n group-IVsubcells where n is an integer greater than or equal to 2, and where thecompositions for the group-IV layers in the group-IV subcells 1, 2, 3, .. . , n may be selected from a group including: Ge, Si, SiGe, CGe, GeSn,SiGeSn, and CSiGeSn.

According to the disclosure, any or all of the two or more group-IVsolar cells may comprise a solar cell emitter in which the majority ofthe total photogeneration in the photovoltaic cell or subcell takesplace, such that the photogenerated current density in the emitter isgreater than that in the base.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: an n-type emitter layer comprising agroup-IV element selected from the group including Ge, Si, SiGe, CGe,GeSn, SiGeSn, CSiGeSn and combinations thereof; and a p-type group-III-Vsemiconductor base, back-surface-field (BSF), or back heterostructurelayer selected from the group including GaAs, AlGaAs, GaInAs, AlGaInAs,InP, GaP, GaInP, GaInPAs, AlInP, AlGaInP, InAs, InPAs, AlInAs, AlAs,GaSb, GaAsSb, InSb, GaInAsSb, GaInNAs, GaInNAsSb, GaN, AlN, InN, GaInN,AlGaN, AlInN, AlGaInN, and combinations thereof.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: an n-type emitter layer comprising agroup-IV element selected from the group consisting of: Ge, Si, SiGe,CGe, GeSn, SiGeSn, CSiGeSn, and combinations thereof; and a p-typegroup-IV semiconductor base, back-surface field (BSF), or backheterostructure layer selected from the group consisting of: Ge, Si,SiGe, CGe, GeSn, SiGeSn, CSiGeSn, and combinations thereof, where thegroup-IV emitter layer has a different semiconductor composition thanthe group-IV base or BSF layer, forming a heterojunction.

Preferably, the heterostructure or heterojunction in any or all of thetwo or more group-IV solar cells comprises materials on both sides ofthe heterostructure or heterojunction, and growth, deposition,formation, and/or annealing conditions at the interface that areselected to reduce minority-carrier recombination at layer interfacesand within the materials on both sides forming the heterostructure orheterojunction.

According to the disclosure, the photovoltaic cell structure may furthercomprise a tunnel junction electrically interconnecting the two or moregroup-IV solar cells grown on, deposited on, formed on, or formed withina growth substrate.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: a first layer comprising a solar cellemitter layer formed from a group-IV layer, and a second layercomprising tunnel junctions formed from one or more III-V semiconductorlayers, where the one or more tunnel junction semiconductor layers maybe selected from a group including: GaAs, AlGaAs, GaInAs, AlGaInAs, InP,GaP, GaInP, GaInPAs, AlInP, AlGaInP, InAs, InPAs, AlInAs, AlAs, GaSb,GaAsSb, InSb, GaInAsSb, GaInNAs, GaInNAsSb, GaN, AlN, InN, GaInN, AlGaN,AlInN, AlGaInN, and combinations thereof.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: a first layer comprising a solar cellemitter layer formed from one or more group-IV semiconductor layers, anda second layer comprising tunnel junctions formed from one or more groupIII-V semiconductor layers, where the tunnel junctions may be selectedfrom a group including:

-   -   p+GaAs/n+GaAs tunnel junctions;    -   p+AlGaAs/n+GaAs tunnel junctions;    -   p+GaAs/n+AlGaAs tunnel junctions;    -   p+AlGaAs/n+AlGaAs tunnel junctions;    -   p+AlGaAs/n+GaInP tunnel junctions;    -   p+AlGaAs/n+AlGaInP tunnel junctions; and    -   p+AlGaInP/n+AlGaInP tunnel junctions,        and where the second layer comprising tunnel junctions may or        may not be positioned adjacent to the first layer.

Furthermore, any or all of the two or more group-IV solar cells maycomprise a solar cell emitter layer formed from one or more group-IVsemiconductor layers that is an epitaxially-grown emitter layercomprising Ge, and that is grown on top of or positioned on top of asecond layer comprising tunnel junctions formed from one or more groupIII-V semiconductor layers, where the tunnel junctions may be selectedfrom a group including:

-   -   p+GaAs/n+GaAs tunnel junctions;    -   p+AlGaAs/n+GaAs tunnel junctions;    -   p+GaAs/n+AlGaAs tunnel junctions;    -   p+AlGaAs/n+AlGaAs tunnel junctions;    -   p+AlGaAs/n+GaInP tunnel junctions;    -   p+AlGaAs/n+AlGaInP tunnel junctions; and    -   p+AlGaInP/n+AlGaInP tunnel junctions,        and where the second layer comprising tunnel junctions may or        may not be positioned adjacent to the first layer.

According to the disclosure, the doping of the solar cell emittercomprising one or more group-IV semiconductor materials in any or all ofthe two or more group-IV solar cells may be influenced by or may consistof the column-III and/or column-V elements in the solar cell base, BSF,and/or tunnel junction layers comprising one or more III-V semiconductorlayers, where the column-III and/or column-V elements act as dopants inthe group-IV emitter.

According to the disclosure, the doping of the solar cell base, BSF, ortunnel junction layers comprising one or more III-V semiconductor layersin any or all of the two or more group-IV solar cells may be influencedby or may consist of the column-IV element or elements in the group-IVsolar cell emitter, where the column-IV element or elements act asdopants in the one or more III-V layers in the solar cell base, BSF,and/or tunnel junction layers.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: a first layer comprising a solar cellemitter layer formed from one or more group-IV semiconductor materials,and a second layer comprising tunnel junctions formed from one or moregroup-IV semiconductor layers, where the one or more tunnel junctionsemiconductor layers may be selected from a group including: Ge, Si,SiGe, CGe, GeSn, SiGeSn, and CSiGeSn.

According to the disclosure, any or all of the two or more group-IVsolar cells may further comprise: a first layer comprising a solar cellemitter layer formed from one or more group-IV semiconductor materials,and a second layer comprising tunnel junctions formed from one or moregroup-IV semiconductor layers, which may be selected from a groupincluding:

-   -   strained or unstrained p+Ge/n+Ge tunnel junctions;    -   strained or unstrained p+SiGe/n+SiGe tunnel junctions;    -   strained or unstrained p+CGe/n+CGe tunnel junctions;    -   strained or unstrained p+SiGeSn/n+SiGeSn tunnel junctions;    -   strained or unstrained p+CSiGeSn/n+CSiGeSn tunnel junctions;    -   strained p+SiGe/n+Ge tunnel junctions;    -   p+Ge/strained n+Si Ge tunnel junctions;    -   strained p+CGe/n+Ge tunnel junctions;    -   p+Ge/strained n+CGe tunnel junctions;    -   p+SiGeSn/n+Ge tunnel junctions;    -   p+Ge/n+SiGeSn tunnel junctions;    -   p+CSiGeSn/n+Ge tunnel junctions; and    -   p+Ge/n+CSiGeSn tunnel junctions.

According to the disclosure, the two or more group-IV solar cells may besubcells within a multijunction photovoltaic cell, and the multijunctionphotovoltaic cell may be selected from the group consisting of a2-junction cell, 3-junction cell, 4-junction cell, 5-junction cell,6-junction cell, and multijunction cells with 7 or more collectingjunctions or subcells—i.e., from the group consisting of n-junctioncells where n is an integer greater than or equal to 2—and themultijunction cell may further comprise a group-IV material to provide aselected bandgap combination among the subcells of the multijunctioncell.

According to the disclosure, any or all of the two or more group-IVsolar cells may be subcells within a multijunction photovoltaic cellthat includes at least one group-IV semiconductor layer, that isepitaxially grown in a deposition apparatus, where the multijunctioncell includes one or more additional subcells comprising III-Vsemiconductor layers.

According to the disclosure, any or all of the two or more group-IVsolar cells may be subcells within a multijunction photovoltaic cellthat includes at least one group-IV semiconductor layer, where themultijunction cell includes one or more additional subcells comprisinggroup-IV semiconductor layers.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer is designed to be positioned closer to the primary lightsource for the solar cell during solar cell operation than the baselayer.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where the baselayer is designed to be positioned closer to the primary light sourcefor the solar cell during solar cell operation than the emitter layer.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer is designed to be positioned at approximately the samedistance from the primary light source for the solar cell during solarcell operation as the base layer.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer is deposited, grown, diffused, or otherwise formed afterthe base layer of the solar cell.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where the baselayer is deposited, grown, diffused, or otherwise formed after theemitter layer of the solar cell.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer is deposited, grown, diffused, or otherwise formed atapproximately the same time as the base layer of the solar cell.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer has a lower energy bandgap than the base layer of thesolar cell.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer has a higher energy bandgap than the base layer of thesolar cell.

According to the disclosure, any or all of the two or more group-IVsolar cells may have an emitter layer and a base layer, where theemitter layer has approximately the same energy bandgap as the baselayer of the solar cell.

Still further, the present disclosure is directed to a photovoltaicelectricity generation system, energy storage system, and/or a vehicularenergy system comprising a photovoltaic cell having a first layercomprising a group-IV material and a second layer comprising a materialselected from the group consisting of: a III-V material and a group-IVmaterial different from the first layer group-IV material, wherein aheterostructure is formed between the first layer and the second layer.

In some alternatives, the photovoltaic cells of the present disclosurehave particular usefulness as a sustainable power source for use instationary or mobile applications including terrestrial andnon-terrestrial applications, and in manned and in unmanned vehicles,including aircraft, spacecraft and surface and sub-surface water-bornevehicles, etc.

Still further, the present disclosure is directed to a terrestrial,non-concentrating or concentrator photovoltaic electricity generationsystem, for utility-scale, commercial, residential, or personalelectricity production.

Still further, the present disclosure is directed to anextraterrestrial, non-concentrating or concentrator photovoltaicelectricity generation system, for satellite, maimed space vehicle, orunmanned space vehicle electric power production in space or near spaceapplications.

BRIEF DESCRIPTION OF THE DRAWING(S)

Having thus described variations of the disclosure in general terms,reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, and wherein:

FIG. 1 is a cross-sectional diagram of a group-IV solar cell where theprimary photoabsorber is an n-type Ge, SiGe, or SiGeSn emitter layerthat may be doped by group-V elements from the III-V semiconductorlayers beneath it and group-V elements from III-V semiconductor layersabove it, and that forms the p-n junction of the solar cell at aheterostructure between the n-type Ge, SiGe or SiGeSn emitter and ap-type GaAs base or BSF layer;

FIG. 2 is a graph showing calculated quantum efficiencies of a thin,epitaxial Ge subcell 4 in a 5-junction cell, and of an optically thicklower Ge substrate cell 5 in a 5-junction cell showing the behavior ofthe resulting current balance among these subcells as a function ofupper subcell thickness;

FIG. 3 is a graph showing calculated densities of both upper and lowergroup-IV subcells from FIG. 2 and the ratio of their currents (J-ratio)plotted as a function of upper group-IV subcell absorber thickness;

FIG. 4 is a cross-sectional diagram of an example group-IV solar cellwhere the primary absorber is an n-type Ge, SiGe or SiGeSn emitter layerthat may be doped by group-V elements from the III-V semiconductorlayers beneath it and group-V elements from III-V semiconductor layersabove it (as well as by group-III elements present), and that forms thep-n junction of the solar cell at a heterostructure between the n-typeGe, SiGe or SiGeSn emitter and the p-type AlGaAs base or BSF layer;

FIG. 5 is a cross-sectional diagram of an example group-IV solar cellwhere the primary absorber is an n-type Ge, SiGe or SiGeSn emitterlayer, forming the p-n junction of the solar cell at a heterostructurewith a general p-type III-V semiconductor base or BSF layer (p-GaAs:Cshown in the figure) and that incorporates a general III-V semiconductorp-type tunnel junction layer (80%-AlGaAs:C shown in the figure) and ageneral III-V semiconductor n-type tunnel junction layer (GaAs:Te shownin the figure);

FIG. 6 is a cross-sectional diagram of an example group-IV solar cellwhere the primary absorber is an n-type Ge, SiGe or SiGeSn emitter layerthat may be doped by group-V elements from the III-V semiconductorlayers beneath it and group-V elements from III-V semiconductor layersabove it, and that forms the p-n junction of the solar cell at aheterostructure between the n-type Ge, SiGe or SiGeSn emitter and thep-type group-IV base or BSF layer with different composition than then-type emitter;

FIG. 7 is a cross-sectional diagram of an example group-IV solar cellwhere the primary absorber is an n-type Ge, SiGe or SiGeSn emitter layerthat may be doped by group-V elements from the III-V semiconductorlayers beneath it and group-V elements from III-V semiconductor layersabove it, and that forms the p-n junction of the solar cell at ahomojunction between the n-type Ge, SiGe or SiGeSn emitter and thep-type group-IV base or BSF layer having substantially the samecomposition as the n-type emitter;

FIG. 8 is a cross-sectional diagram of an example group-IV solar cellwhere the primary absorber is an n-type Ge, SiGe or SiGeSn emitter layerthat may be doped by group-V elements from the III-V semiconductorlayers beneath it and group-V elements from III-V semiconductor layersabove it, and where an upper epitaxial IV cell is interconnected inseries to a lower substrate group-IV solar cell with a group-IV tunneljunction;

FIG. 9 is a cross-sectional diagram of an example upper group-IV solarcell, such as, for example, an epitaxial Ge solar cell interconnected toa second, lower group-IV cell via a tunnel junction, where the p-typeside of the tunnel junction comprises a group-IV semiconductor having adifferent composition than the group-IV emitter, base, BSF or n-typeside of the tunnel junction, thereby forming a heterostructure;

FIG. 10 is a cross-sectional diagram of an example upper group-IV solarcell, such as, for example, an epitaxial Ge solar cell interconnected toa second, lower group-IV cell via a tunnel junction, where the n-typeside of the tunnel junction comprises a group-IV semiconductor having adifferent composition than the group-IV emitter, base, BSF or p-typeside of the tunnel junction, thereby forming a hetero structure;

FIG. 11 is a cross-sectional diagram of an example upper group-IV solarcell, such as, for example, an epitaxial Ge solar cell with a windowlayer comprising a group-IV semiconductor such as, for example, Ge,SiGe, CGe, SiGeSn and/or CSiGeSn with a different semiconductorcomposition than the emitter, forming a heterostructure;

FIG. 12 is a cross-sectional diagram of an example solar cell structure,where an upper group-IV subcell has a higher bandgap than a lowergroup-IV subcell, optionally allowing the upper subcell to be opticallythicker than if both subcells had the same bandgap, and optionallyallowing part of the upper group-IV subcell to be an undiffused p-typebase in the finished device. Examples include using SiGe, CGe, SiGeSn,or CSiGeSn for the main absorber layer of the upper subcell, and Ge forthe lower subcell;

FIG. 13 is a graph plotting experimental, measured light I-V curves of a2-junction (2J) Ge/Ge subcell with an interconnecting AlGaAs/Ga?Astunnel junction with both subcells active in a structure similar to thatshown in FIG. 4 with an epitaxially-grown Ge absorber layer in the uppergroup-IV subcell, showing the voltage addition associated with theseries interconnection of both active group-IV subcells. Asingle-junction (1J) Ge subcell formed in the Ge substrate is alsoplotted for comparison. The 2-junction Ge/Ge cell structure results inapproximately twice the voltage of the 1-junction Ge cell, with over 0.4V higher open-circuit voltage V_(oc) for the 2J Ge/Ge cell compared tothe 1J cell;

FIG. 14 is a cross-sectional diagram and graph showing calculatedsubcell and multijunction cell light I-V curves and calculated light I-Vparameters for a 5-junction (5J) AlGaInP/AlGaInAs/GaInAs/Ge/Ge solarcell using an epitaxial Ge cell 4 and a 2-junction Ge/Ge combination forthe bottom two cell layers;

FIG. 15 cross-sectional diagram and graph showing calculated subcell andmultijunction cell light I-V curves and calculated light I-V parametersfor a 5-junction (5J) with an epitaxial Ge cell 4 and a 2-junction Ge/Gecell configuration at the bottom of the stack, and with energy wells tolower the effective bandgap of subcell 3;

FIG. 16 is a cross-sectional diagram and graph showing calculatedsubcell and multijunction cell light I-V curves and calculated light I-Vparameters for a 5-junction (5J) cell using an epitaxial Ge cell 4 and a2-junction Ge/Ge cell configuration at the bottom of the stack, and witha metamorphic structure to lower the bandgap of subcell 3;

FIG. 17 is a graph showing calculated external quantum efficiency (EQE),internal quantum efficiency (IQE), and an overall absorptance for a5-junction cell incorporating an epitaxial heterojunction Ge cell 4,with an AlGaInP/AlGaInAs/GaInAs/epitaxial Ge/ substrate Ge cellstructure, with a base bandgap combination of 2.05/1.68/1.41/0.67/0.67eV;

FIG. 18 is a graph showing measured light I-V characteristics of twofully-integrated prototype 5-junction (5J) cells, incorporating aheterojunction epitaxial Ge cell 4, with structure as shown in thecross-sectional cell schematic. 4-junction (4J) cells with an identicalstructure except that the epitaxial Ge cell 4 and associated tunneljunction are absent are also shown for comparison. The epitaxial Ge cell4 in the 5J cells results in an ˜0.4 V higher open-circuit voltageV_(oc) for the 5J cells compared to the 4J cells.

DETAILED DESCRIPTION

According to the present disclosure, systems and methods are disclosedfor making significant improvements for terrestrial and non-terrestrialphotovoltaic cells, such as, for example, concentrator solar cells andspace solar cells, providing significantly higher efficiency than areavailable in today's most advanced solar cells. The approaches describedherein make use of the lower-cost, more reliable, and more scalableprocesses of upright solar cell growth, and lattice-matched growth ormetamorphic growth with small lattice mismatch, as opposed to theinverted cell growth and cell processing, and metamorphic growth withhigh lattice mismatch of inverted metamorphic cells.

The present disclosure allows the formation of multijunction cells, suchas, for example, 3-junction (3J), 4-junction (4J), 5-junction (5J),6-junction (6J), 7-junction (7J) solar cells, or cells with 8 junctionsor more, that incorporate a group-IV solar cell.

Preferred cells comprise, for example, epitaxial Ge, Si, SiGe, CGe,GeSn, SiGeSn, CSiGe, CGeSn, CSiGeSn, etc. to provide the desired,pre-selected subcell bandgap combination, thus increasing the efficiencyof the multijunction cell. Such group-IV solar cells at least one layercomprising at least one element from column IV (four) of the periodictable of elements, e.g., carbon (C), silicon (Si), germanium (Ge), andtin (Sn).

The group-IV layer in the solar cell may be a primary photogenerationlayer, or primary absorber layer, such that the majority of chargecarrier photogeneration for the solar cell takes place in the primaryabsorber layer.

The approaches described herein make use of the low-cost, reliable, andscalable processes of upright solar cell growth, and lattice-matchedgrowth or metamorphic growth with small lattice mismatch. Theseprocesses have certain advantages compared with inverted cell growth andcell processing, and metamorphic growth with high lattice mismatch,associated with other high-efficiency solar cell architectures, such as,for instance, inverted metamorphic cells.

According to the present disclosure, the heterostructure layers aredesigned to reduce minority-carrier recombination at interfaces, and bemore transparent to light in layers where photogeneration isundesirable, such as, for example, tunnel junction layers, windowlayers, base or BSF layers, and other layers in which the collectionprobability of photogenerated carriers is low.

Further, according to the present disclosure, dopant or other impuritydiffusion from one layer to another is engineered or managed. In caseswhere it is desirable, doping of group-IV semiconductor layers by nearbyIII-V semiconductors, and doping of III-V semiconductor layers by nearbygroup-IV semiconductors, is used to form the final doping of some solarcell layers. In other cases for which doping caused by adjacent layersis undesirable, dopant or other impurity diffusion from one layer toanother is inhibited by choice of the layer structure, compositions,growth conditions, and processing conditions.

Moreover, according to the present disclosure, when such group-IV layersare incorporated into multijunction cells with III-V semiconductorlayers, the group-III and group-V elements in the III-V semiconductorswill typically dope nearby group-IV layers. Similarly, the group-IVelements will dope the III-V semiconductor layers. Such doping betweensemiconductor layers from different columns of the periodic table,referred to herein as cross-doping, is often difficult to avoid andresults in compromised cell efficiency for some cell designs. However,the cell designs disclosed herein use particular groupings ofmultijunction solar cell device design architectures with regard to:n-type or p-type doping type and doping concentration, semiconductorfamily (group-IV semiconductors, III-V semiconductors, II-VIsemiconductors, etc.), semiconductor material compositions and latticeconstant, location in the multijunction stack, selection of adjacentsemiconductor layers, semiconductor growth conditions and thermal budgetand annealing conditions that influence the incorporation and movementof dopant atoms, crystal lattice strain, relaxation, and dislocations,and other parameters as described in the present disclosure toaccommodate such cross-doping and use it to advantage to result in thefinal doping of the solar cell layers, resulting in a solar cell havingsignificantly improved voltage and efficiency.

In addition, according to the present disclosure, better use is alsomade of the excess photogenerated current in the Ge bottom subcell ascompared with conventional lattice-matched GaInP/GaInAs/Ge 3-junctionsolar cells. Another cell type that makes better use of the excessphotogenerated current density in the Ge bottom subcell of conventional3J cells is represented by inverted metamorphic solar cells such asGaInP/GaAs/GaInAs 3J cells. However, this approach has significantlyhigher growth cost and processing cost than the upright processing andlattice-matched growth approaches described and disclosed herein. Forexample, the present disclosure also contemplates, for example, usingdilute nitride GaInNAs(Sb) subcells with ˜1-eV bandgap in multijunctioncells as another method for achieving a better division of the solarspectrum in the long wavelength range of the solar spectrum. However,such dilute-nitride cells have historically had unacceptably low currentdensity, voltage, or both to be used in this way, unless higher costgrowth methods are used.

The term group-IV solar cell refers to a solar cell having at least onelayer comprising one or more group-IV elements, e.g., C, Si, Ge, and Sn,individually, or in combination with other elements. The group-IV solarcell layer may be selected, for instance, from a group including thesemiconductor compositions of Ge, Si, SiGe, CGe, GeSn, SiGeSn, CSiGe,CGeSn, or CSiGeSn. The GaInP nucleation layer on the group-IV solar cellin many of the diagrams below can be used as a window layer on the cellin addition to serving as a nucleation layer. The abbreviations C1, C2,C3, C4, C5, etc. refer to subcell or cell 1 (the top subcell), andsubcells or cells 2, 3, 4, 5, etc., respectively, beneath the top cellin normal operation of a multijunction solar cell. The abbreviations 1J,2J, 3J, 4J, 5J, 6J, 7J, etc, refer to 1-junction, 2-junction,3-junction, 4-junction, 5-junction, or 6-junction, 7-junction, etc.,solar cells.

Some variants of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all variations of the disclosure are shown. Indeed, the presentdisclosure may be embodied in many different forms and should not beconstrued as limited to the variations set forth herein. Instead, theseillustrative variations are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the disclosureto those skilled in the art. For example, unless otherwise indicated,referencing something as being a first, second or the like should not beconstrued to imply a particular order. Also, something may be describedas being “above” something else and, unless otherwise indicated, mayinstead be “below”, and vice versa. Similarly, something described asbeing to the left of something else may instead be to the right, andvice versa. Like reference numerals refer to like elements throughout.

Therefore, preferred embodiments according to the present disclosureinclude a group-IV solar cell that is optically thin in order to leak asubstantial, amount of light to an additional subcell layer beneath,such as, for example, an epitaxially-grown Ge, SiGe, CGe, GeSn, SiGeSn,or CSiGeSn subcell on top of a Ge substrate subcell in a multijunctionsolar cell, in which most or substantially all of the photoabsorbinggroup-IV layer in the solar cell is doped n-type by the growth of III-Vsubcells on top and/or by III-V semiconductor layers on which thegroup-IV solar cell is grown, such that the p-n junction is formed atthe back of the main photoabsorbing group-IV layer.

An optically thin solar cell is one which transmits a non-zero amount ofthe light incident upon it to layers beneath the solar cell, i.e., tolayers on the opposite side of the subcell from the light source.Typically, an optically thin subcell will transmit an amount of incidentlight in the range of approximately 5% to 90% of the light intensity orequivalent photogenerated current density that could in principle beused by a solar cell with that bandgap. The light intensity orequivalent photogenerated current density that could in principle beused by a solar cell with a given bandgap, is the light in the spectrumincident on the cell—including the effects of any light filtering abovethe cell, for instance, from other subcells in a multijunction solarcell stack—with photon energy high enough to produce carrierphotogeneration in the solar cell, i.e., higher than approximately thesolar cell bandgap. More typically, the range of the amount of lighttransmitted by an optically thin solar cell is approximately 10% to 70%,and will depend on the desired use of the light transmitted by the solarcell, for instance, by the specific bandgap combination of subcells in amultijunction cell beneath the optically thin subcell. For many of thesolar cell designs in the present disclosure, in which an upper group-IVsolar cell and a lower group-IV solar cell are to share the availablephotogenerated current density equally, in order for the upper and lowersubcells to be current matched in a multijunction solar cell, the upper,optically thin cell will preferably have a range of the amount oftransmitted light of approximately 40% to 65%, and more preferably 45%to 60%, and still more preferably 48% to 56%. The desired amount oflight to be transmitted by the optically thin upper subcell depends onnon-ideal effects in the solar cell structure, such as absorption byintervening layers like the tunnel junction layers between the upper andlower subcell, and incomplete collection of photogenerated carriers inthe lower subcell.

Further examples include a group-IV photovoltaic cell, such as, forexample a solar cell where the main or primary photoabsorbing layer isan epitaxially-grown Ge, SiGe, CGe, GeSn, SiGeSn, or CSiGeSn n-typeemitter layer, doped n-type by the nucleation and growth of III-Vsemiconductors on top of it, from which photogenerated minority holesare collected at a p-n junction at the back of this n-type Ge, SiGe,CGe, GeSn, SiGeSn, or CSiGeSn emitter layer.

In addition, the present disclosure contemplates a photovoltaic cellarrangement with the n-type Ge, SiGe, CGe, GeSn, SiGeSn, or CSiGeSnemitter solar cell described above, and preferably having a thickness inthe range of from about 0.01 to about 10 microns, more preferably in therange of from about 0.1 to about 2 microns, and still more preferably inthe range of from about 0.3 to about 0.7 microns.

Further, the present disclosure contemplates a photovoltaic cellarrangement with the n-type Ge, SiGe, CGe, GeSn, SiGeSn, or CSiGeSnemitter solar cell described above, and where the p-n junction at theback of the emitter layer is formed by a p-type III-V layer that rendersineffective the n-type doping that diffuses in from the front of thecell (since the group-V elements that cause n-type doping and group-IIIelements that cause p-type doping in group-IV semiconductors are notdopants in III-V semiconductors). As shown in FIG. 1, the n-type Ge,SiGe, CGe, GeSn, SiGeSn, or CSiGeSn emitter solar cell is located abovethe lower subcell, where above indicates a direction toward the primarylight source for the solar cell, e.g., the sun, where the p-n junctionat the back of the emitter layer is a heterojunction between the n-typegroup-IV emitter and a p-type III-V semiconductor base,back-surface-field (BSF), or back-heterostructure layer comprising aIII-V semiconductor such as, but not limited to, GaAs, AlGaAs, GaInAs,GaInP, AlGaInAs, and AlGaInP. The n-type group-IV layer may be doped bygroup-V elements from III-V semiconductor layers beneath it and group-Velements from III-V semiconductors above it, and the n-type group-IVemitter layer forms the p-n junction of the solar cell at aheterostructure between the n-type Ge, SiGe, CGe, GeSn, SiGeSn, orCSiGeSn emitter and p-type GaAs base or BSF layer. Furthermore, theheterostructure formed between the group-IV emitter and the III-Vsemiconductor base, BSF, or back-heterostructure in FIG. 1 is across-column heterostructure.

The III-V semiconductor base, BSF, or back-heterostructure layermaterial, and indeed the material of any or all the III-V semiconductorlayers described in various configurations the semiconductor devices ofthe present disclosure, may be chosen from a preferred group including,but not limited to: GaAs, AlGaAs, GaInAs, AlGaInAs, InP, GaP, GaInP,GaInPAs, AlInP, AlGaInP, InAs, InPAs, AlInAs, AlAs, GaSb, GaAsSb, InSb,GaInAsSb, GaInNAs, GaInNAsSb, GaN, MN, InN, GaInN, AlGaN, AlInN,AlGaInN, and combinations thereof.

In general, a back-surface-field (BSF) layer may have the same bandgapas the base layer, or it may have a different bandgap than the baselayer, in which case the BSF layer is also a back-heterostructure layer.The solar cell BSF layer typically has the same doping type as the solarcell base. The BSF layer is often, but not always, doped at a higherdoping concentration with respect to the base layer in order to suppressthe concentration of minority-carriers and suppress their rate ofrecombination in the BSF layer. The BSF layer often, but not always, hasa higher bandgap with respect to the base layer in order to suppress theconcentration of minority-carriers and suppress their rate ofrecombination in the BSF layer.

In general, a cross-column heterostructure is a heterostructure formedbetween: a first semiconductor layer on a first side of theheterostructure comprising a first family of semiconductors formed fromelements from a given column or set of columns of the periodic table ofelements, and; a second semiconductor layer on the opposite, second sideof the heterostructure comprising a second family of semiconductorsdifferent from the first family of semiconductors of the firstsemiconductor layer, formed from elements from a column or different setof columns of the periodic table, where the family of semiconductors foreach layer may be selected from a group including, but not limited to:group-IV semiconductors; III-V semiconductors; II-VI semiconductors;semiconductors; II-IV-VI semiconductors; and other families, where theroman numerals refer to the number of a column of elements in theperiodic table of elements.

Still further, the present disclosure contemplates an upper group-IVsolar cell that is optically thin, that transmits roughly half of thelight energy incident upon it to a second lower group-IV solar cellpositioned beneath the first, upper group-IV cell, such that it is inoptical series with the first cell. The upper group-IV cell and thelower group-IV cell may be connected in electrical series by a tunneljunction preferably made as transparent as possible to the wavelengthsused by the second, lower cell. As shown in FIG. 2, calculated quantumefficiencies of a thin, upper, epitaxial Ge subcell 4 in a 5J cell, andof an optically thick, lower Ge substrate subcell 5 in a 5J cell, areplotted as a function of wavelength for various upper subcellthicknesses using Ge absorption coefficients. The behavior of theresulting current balance among these subcells for various upper subcellthicknesses can be seen in FIG. 2 as a function of upper subcellthickness. The current densities of both the upper and lower group-IVsubcells from FIG. 2, and the ratio of their currents, called theJ-ratio, is plotted as a function of upper group-IV subcell absorberthicknesses in FIG. 3.

FIG. 4 shows a cross-sectional diagram of an example -IV solar cell forwhich the primary photoabsorber is an n-type Ge, SiGe, or SiGeSn emitterlayer, which may be doped by group-V elements from III-V semiconductorlayers beneath it and group-V elements from III-V semiconductors aboveit, and which forms the p-n junction of the solar cell at aheterostructure between the n-type Ge, SiGe, or SiGeSn emitter andp-type AlGaAs base or BSF layer. The high p-type carbon doping level inthe 1×1020 cm⁻³ range that is readily achievable in AlGaAs, (as comparedto GaAs), is beneficial to that structure, since highly doped p-typelayers are more resistant to unwanted doping from Ge diffusing in fromnearby Ge layers. Such an arrangement maintains a solar cell p-njunction between the n-Ge and p-AlGaAs, and maintains a tunnel junctionwith high doping on both sides of, for instance, a p-AlGaAs/n-GaAstunnel junction.

FIG. 5 shows another preferred arrangement with the n-type Ge, SiGe, orSiGeSn emitter solar cell described above, that may be doped by group-Velements from III-V semiconductor layers beneath it and group-V elementsfrom III-V semiconductors above it, where the p-n junction at the backof the emitter layer is a heterojunction between the n-type group-IVemitter and a p-type III-V semiconductor base or BSF layer, such asGaAs, AlGaAs, GaInP, or AlGaInP, and which incorporates a III-V tunneljunction for which the p-type side of the tunnel junction may becomposed of a III-V semiconductor such as GaAs, AlGaAs, GaInP orAlGaInP, and for which the n-type side of the tunnel junction may alsobe composed of a III-V semiconductor such as GaAs, AlGaAs, GaInP, orAlGaInP. One example is shown in FIG. 5, with an n-type Ge emitter andmain absorber layer, a p-type GaAs base or BSF layer, and an AlGaAs:Cp-type tunnel layer, and a GaAs:Te n-type tunnel layer. Where AlGaAslayers are used, the high p-type carbon doping level in the 1×10²⁰ cm⁻³range that is readily achievable in AlGaAs, (as compared to GaAs), isbeneficial to the structure, since highly doped p-type layers are moreresistant to unwanted dopant compensation from Ge diffusing in fromnearby Ge layers, for instance, thus maintaining high doping levels onboth sides of the p-AlGaAs/n-GaAs tunnel junction shown in FIG. 5.

According to contemplated alternatives, a solar cell structure where thetunnel junction between an upper group-IV subcell (C4), such as anepitaxial Ge subcell, and a lower group-IV subcell formed from thegrowth substrate (C5), such as a Ge growth substrate, is composed ofIII-V semiconductors, such as a p+GaAs/n+GaAs tunnel junction, ap+AlGaAs/n+GaAs tunnel, a p+AlGaAs/n+AlGaAs tunnel, p+AlGaAs/n+GaInPtunnel, as shown in FIGS. 1, 4, and 5.

The n-type Ge, SiGe, or SiGeSn emitter solar cell described above, wherethe p-type III-V semiconductor base or BSF layer is chosen to includespecific group-III or group-V dopants in the group-IV solar cell on topof it that are desirable for the functioning of the group-IV solar cell,or to exclude specific group-III or group-V dopants in the group-IVsolar cell on top of it that are undesirable for the functioning of thegroup-IV solar cell.

Further, the n-type Ge, SiGe, or SiGeSn emitter solar cell describedabove, where the p-type III-V semiconductor base or BSF layer is chosenfrom a preferred list including GaAs, AlGaAs, GaInAs, AlGaInAs, InP,GaP, GaInP, GaInPAs, InAs InPAs, AlInAs, AlAs, GaSb, GaAsSb, InSb,GaInAsSb, GaInNAs, GaInNAsSb and combinations thereof

FIG. 6 shows still another preferred arrangement, with the n-type Ge,SiGe, or SiGeSn emitter solar cell described above, where the p-njunction at the back of the emitter layer is a heterojunction betweenthe n-type group-IV emitter and a p-type group-IV base or BSF layer withdifferent semiconductor composition than the group-IV emitter, such asan epitaxial n-type Ge emitter and main photoabsorber layer on astrained p-type SiGe base or BSF layer, or an epitaxial n-type Geemitter on an unstrained p-type SiGeSn base or BSF with wider bandgapthan the Ge emitter. More specifically, FIG. 6 shows (in across-sectional diagram) a group-IV solar cell for which the primaryphotoabsorber is an n-type Ge, SiGe, or SiGeSn emitter layer, that maybe doped by group-V elements from III-V semiconductor layers beneath itand group-V elements from III-V semiconductors above it, and which formsthe p-n junction of the solar cell at a heterostructure between then-type Ge, SiGe, or SiGeSn emitter and p-type group-IV base or BSF layerwith different composition than the n-type emitter.

According to the present disclosure, a group-IV solar cell structure,where the p-type group-IV base or BSF layer with different semiconductorcomposition than the group-IV emitter results in: reducedminority-carrier recombination at an interface; higher cell voltage;greater transparency to wavelengths that are not desirable to beabsorbed; reduced undesirable cross-column doping among nearbysemiconductor layers by eliminating unwanted dopant species in III-V orgroup-IV layers; enhanced desirable cross-column doping among nearbysemiconductor layers by including desired dopant species in III-V orgroup-IV layers; and/or improved dopant confinement, e.g., as a dopantdiffusion barrier, or as a trap for dopant species.

According to one variant, an epitaxially-grown SiGe or SiGeSnheterostructure back-surface field (BSF) layer disposed on a Ge or SiGesolar cell formed by epitaxial growth or from a Ge or SiGe substratewafer.

Further, an epitaxially-grown SiGe or SiGeSn heterostructure layer usedin a tunnel junction for electrical connection to a solar cell, wherethe heterostructure layer has higher band gap than one or more layers inthe solar cell to which it connects, or than other layer(s) in thetunnel junction structure. According to another contemplatedalternative, a window, emitter, BSF, or tunnel junction heterostructurelayer formed from a III-V semiconductor where one or more adjacent solarcell or tunnel junction layers is prefercomposed of a group-IVsemiconductor such as Ge, Si, SiGe, or SiGeSn.

In addition, the present disclosure contemplates an epitaxially-grownSiGe or SiGeSn heterostructure window or emitter layer disposed on a Geor SiGe solar cell formed by epitaxial growth or formed from a Ge orSiGe substrate wafer, and an epitaxially-grown heterostructure layerwith higher band gap and different lattice constant than one or moreadjacent layers, with thickness and composition such that theheterostructure layer remains pseudomorphic with respect to adjacentlayer(s), i.e., such that the heterostructure layer is strained and notrelaxed, with an intact, largely defect-free crystal structure. Thesignificance of variations using one or more strained layers in thepresent invention, compared to the variations of the present inventionusing unstrained layers alone, are several fold. The use of strainedlayers allows the use of different semiconductor compositions than wouldbe available with only unstrained layers, increasing the range ofbandgaps, light absorption characteristics, electronic properties suchas minority-carrier lifetime and mobility, and densities of states inthe valence and conduction bands, and other parameters available for themultijunction cell layers, which can be used to increase multijunctionsolar cell performance and that of their subcells, including energyconversion efficiency and ease of manufacture, as well as theperformance of other semiconductor devices. The strain in semiconductorlayers itself also changes the energy band structure of semiconductorsand other electronic properties such as lifetime, mobility, anddensities of states, and increases the range of energy band offsets andtypes of band lineups (type I heterojunctions with nested band lineup,type II heterojunctions with staggered band lineup, etc.) in theconduction and valence bands, which are also available with unstrainedheterojunction layers by changing composition but with a smaller rangeof options in the unstrained layer case, all of which may be used toincrease the performance of subcells and multijunction solar cells, aswell as of other semiconductor devices. The strain in semiconductorlayers may also inhibit or enhance the incorporation of dopant atoms inthe layer, may can also inhibit or enhance the diffusion of dopant atomsin the layer, may inhibit or enhance the tendency of some dopants toremain on the growth surface during epitaxial growth rather than beingincorporated into the semiconductor layer, and may inhibit or enhancethe generation, annihilation, and transport of point defects in thecrystal lattice, which also influence the movement of dopant atoms aswell as the final crystal quality and dislocation density of thesemiconductor crystal, all of which may be used to increase theperformance of subcells and multijunction solar cells, as well as ofother semiconductor devices.

A still further preferred arrangement contemplated by the presentdisclosure is shown in FIG. 7 with an n-type Ge, SiGe, or SiGeSn emittersolar cell described above, where the p-n junction at the back of theemitter layer is a homojunction between the n-type group-IV emitter (Ge,SiGe, or SiGeSn) and a p-type group-IV base layer, with the samecomposition as the emitter layer. The p-type group-IV base layer may bedoped during epitaxial growth, or in may be p-type doped by thediffusion of group-III elements from the III-V layers beneath the cell.Group-III diffusions into Ge tend to have narrower profiles and higherdoping concentration than group-V diffusions, aiding the formation of ap-type homojunction layer.

Further, the n-type Ge, SiGe, SiGeSn solar cell described above, wherethe p-type III-V semiconductor BSF or tunnel junction layer compositionat the back of the cell, upon which the group-IV solar cell is grown, ischosen to optimize a p-type layer that forms at the back of the group-IVlayer and creates a p-n junction in the group-IV solar cell, forinstance by choosing the p-type III-V layer to contain B, Al, Ga, In, orother group-III element, or to specifically exclude these elements, inorder to achieve the desired properties of the p-type diffused region atthe back of the group-IV cell, e.g., high doping or a narrow dopingprofile.

According to another preferred variant, the present disclosurecontemplates an n-type Ge, SiGe, SiGeSn solar cell described above,where the p-n junction at the back of the emitter layer is ahomojunction between an n-type III-V semiconductor layer and a p-typeIII-V semiconductor layer at the back of the group-IV solar cell. In apreferred embodiment, the majority of the photo generation in thesubcell occurs in an n-type group-IV layer of the group-IV solar cell,an n-type III-V semiconductor layer contacts the back (the side awayfrom the primary light source) of the first n-type group-IV layer, and ap-type III-V semiconductor layer contacts the back of the aforementionedn-type III-V semiconductor layer, forming the p-n junction of thegroup-IV solar cell (a solar cell with a group-IV absorber) between then-type and the p-type III-V semiconductor layers.

In yet another preferred arrangement, as shown in FIG. 8, the presentdisclosure further contemplates a solar cell structure where the tunneljunction between an upper group-IV subcell (C4), such as, for example,an epitaxial Ge subcell, and a lower group-IV subcell formed from thegrowth substrate (C5), such as a Ge substrate, is composed of group-IVmaterials, such as a p+Ge/n+Ge tunnel junction, a p+SiGe/n+Ge tunnel, ap+Ge/n+SiGe tunnel, p+SiGeSn/n+Ge tunnel, or a p+Ge/n+SiGeSn tunnel. Inthis arrangement, the primary photoabsorber is an n-type Ge, SiGe, orSiGeSn emitter layer, which may be doped by group-V elements from III-Vsemiconductor layers beneath it and group-V elements from III-Vsemiconductors above it, and for which an upper epitaxial group-IV cellis interconnected in series to a lower substrate group-IV solar cellwith a group-IV tunnel junction.

FIG. 9 shows yet another preferred arrangement contemplated by thepresent disclosure with a group-IV solar cell structure in which thep-type side of the tunnel junction is a group-IV semiconductor such as,for example, Ge, SiGe, CGe, SiGeSn, or

CSiGeSn with a different semiconductor composition than the emitter,base, BSF, or n-type side of the tunnel junction, forming aheterostructure solar cell. The n-type side of the tunnel junction maycomprise a group-IV semiconductor or a III-V semiconductor material.Examples include a strained pseudomorphic SiGe, strained pseudomorphicCGe, or lattice-matched SiGeSn p-type tunnel junction layer, with higherbandgap than a Ge primary absorber layer.

According to still another alternative, the present disclosurecontemplates, a group-IV solar cell structure, where the p-type group-IVtunnel junction layer with different semiconductor composition than thegroup-IV emitter results in reduced minority-carrier recombination at aninterface, higher cell voltage, greater transparency to wavelengths thatare not desirable to be absorbed, and/or improved dopant confinement,e.g., as a dopant diffusion barrier, or as a trap for dopant species.

FIG. 10 shows yet another preferred arrangement contemplated by thepresent disclosure with a group-IV solar cell structure in which then-type side of the tunnel junction is a group-IV semiconductor such as,for example, Ge, SiGe, CGe, SiGeSn, or CSiGeSn with a differentsemiconductor composition than the emitter, base, BSF, or p-type side ofthe tunnel junction, forming a heterostructure solar cell. The p-typeside of the tunnel junction may comprise a group-IV semiconductor or aIII-V semiconductor material. Examples include a strained pseudomorphicSiGe, strained pseudomorphic CGe, or lattice-matched SiGeSn n-typetunnel junction layer, with higher bandgap than a Ge primary absorberlayer.

FIG. 11 shows yet another preferred arrangement contemplated by thepresent disclosure with a group-IV solar cell structure in which thewindow layer is a group-IV semiconductor such as Ge, SiGe, CGe, SiGeSn,or CSiGeSn with a different semiconductor composition than the emitter,forming a heterostructure solar cell. The window layer may also bepositioned on the base, in which case the window also forms the emitterof the cell, forming a heteroemitter solar cell. Examples include astrained pseudomorphic SiGe, strained pseudomorphic CGe, orlattice-matched SiGeSn n-type tunnel junction layer, with higher bandgapthan a Ge primary absorber layer. The group-IV window layer may also bepositioned on the emitter of the lower group-IV solar cell, or directlyon the base of the upper and/or lower group-IV solar cells.

According to the present disclosure, advantages of the exampleheterostructures in FIGS. 1, 4-6, and 8-11 include: reducedminority-carrier recombination at interfaces; increased cell voltage;the ability to control light reflectance at interfaces; increasedtransmittance and lower wasted light absorption in inactive tunneljunction layers, and inactive window and BSF layers, and/or improveddopant confinement, e.g., as a dopant diffusion barrier, or as a trapfor dopant species, to keep doping concentrations high in window and BSFlayers, to keep doping profiles high and abrupt in tunnel junctions, andto reduce unwanted dopants elsewhere. The GaInP nucleation layer growthon a Ge subcell layer, such as an epitaxial Ge layer, also forms aheterostructure window layer itself on the Ge subcell. The absence of Asin a GaInP, AlInP, or AlGaInP window and nucleation layer, and the lowerdiffusion coefficient for P compared to As, limit the extent to whichn-type dopant diffuses into the Ge, SiGe, SiGeSn, or other group-IVsolar cell layer beneath.

In a still further variation, the present disclosure contemplates agroup-IV solar cell with a strained SiGe heteroemitter/window layer ontop of it which, in addition to its minority-carrier recombinationsuppression and high light transmittance functions, also suppressesdiffusion of group-V dopants from the III-V growth on top into thegroup-IV solar cell, allowing the group-IV solar cell to be opticallythin and still have the p-n junction toward the front of the cell.

In another alternative, a group-IV solar cell is disclosed having a lowtemperature III-V layer deposited on top, from which a reduced amount ofgroup-V elements diffuse into the group-IV solar cell due to the lowertemperature, and such that the low temperature III-V layer acts as adopant diffusion barrier for group-V elements from the growth of III-Vlayers at higher temperature later in the growth sequence. Here, a lowtemperature layer is a layer grown or deposited at a temperature that is20 degrees Celsius or more lower than the layers grown or depositedlater in the growth or deposition sequence.

Further, a group-IV solar cell is disclosed with an Al-containing III-Vlayer deposited on top which acts as a dopant diffusion barrier forgroup-V elements from the growth of III-V layers later in the growthsequence. In this case, the Al-containing layer has a lower diffusioncoefficient, for the group-V elements for which the Al-containing layeracts as a diffusion barrier, than a material composed of the samematerial but without any aluminum (Al) present in the composition.

FIG. 12 shows yet another preferred arrangement contemplated by thepresent disclosure with a solar cell structure in which the uppergroup-IV subcell has a higher bandgap than the lower group-IV subcell,allowing the upper cell to be optically thicker than if both subcellshad the same bandgap, while still transmitting significant light to thesubcell beneath. This in turn allows the p-n junction to be a smallerfraction of the total layer thickness from the front of the cell, giventhat the group-V n-type dopants diffuse approximately the same depthinto the group-IV photoabsorption layer. Examples include using SiGe,CGe, SiGeSn, or CSiGeSn for the main absorber layer of the uppersubcell, and Ge for the lower subcell.

The present disclosure contemplates another set of examples includesusing Ge, SiGe, SiGeSn, or CSiGeSn, for the upper subcell, and asemiconductor with smaller bandgap in the lower subcell (smaller thanthe bandgap of Ge in the case of a Ge upper subcell). SiGe will be intensile strain if used in combination with other subcells at the Gelattice constant, which can cause SiGe to relax its crystal lattice andform dislocations if the Si composition is high or the SiGe layer isthick. However, the Si composition can be kept low, the SiGe layers canbe used in strain-balanced configurations, or the SiGe layers can becombined with solar cells at a lower lattice constant than that of Ge tokeep the SiGe layers from relaxing. SiGeSn and CSiGeSn can be grownlattice-matched to Ge.

According to the disclosure, a feature of an optically thicker uppergroup-IV subcell, of ˜1-5 microns in thickness, is that the n-typediffusion of group-V elements from III-V semiconductor growth on top ofthe group-IV subcell will typically be only 0.4-0.6 microns in depth,allowing part of the group-IV subcell to be an undiffused p-type base inthe finished device, as in more conventional solar cell structures, andas also shown in FIG. 12.

FIG. 13 plots experimental, measured light I-V curves of a 2-junction(2J) Ge/Ge subcell, or 2× Ge subcell, with an interconnectingAlGaAs/GaAs tunnel junction and with both subcells active, and having astructure similar to that shown in FIG. 4 with an epitaxially-grown Geabsorber layer in the upper group-IV subcell, that is largely orentirely n-type due to group-V element diffusion from III-Vsemiconductor layers on both top and bottom. The upper 3 subcells inFIG. 4 were replaced in this experimental sample with growth ofinactive, isotype solar cell layers in order to focus on the performanceof the 2J Ge/Ge cell. The inactive, isotype layers used give the fullthermal budget and full light filtering properties that would come withactive upper subcells. The light I-V measurements were made on modifiedterrestrial quick process (MTQP) solar cells, under a pulsed solarsimulator at a light intensity greater than 2000 suns (200.0 W/cm²). Thedual junction epitaxial Ge/substrate Ge cell pair developed a highopen-circuit voltage Voc of 0.87 V at this concentration. The light I-Vcharacteristic for a single junction Ge subcell formed in the Gesubstrate, with the same inactive, isotype upper cell layers that wereused in the 2J Ge/Ge cell case, is also plotted in FIG. 13 forcomparison. Under the same test conditions, this single junction Ge cellhas a Voc of 0.44 V. Thus, the 2-junction Ge/Ge cell structure, with aheterojunction, epitaxially-grown Ge subcell, interconnected by a tunneljunction to another active Ge subcell formed in the Ge growth substrate,delivers approximately twice the voltage of a 1-junction Ge cell. Thefunctioning epitaxial Ge solar cell in a dual junction Ge/Ge cellconfiguration, in combination with functioning tunnel junctions at highcurrent densities, enables high-efficiency cells with 4, 5, 6, or morejunctions with an epitaxial Ge subcell.

According to the present disclosure, preferred group-IV solar cellsemploy a heterostructure layer such as those described earlier in thetext, where the solar cell is a subcell that is part of a multijunctionsolar cell. The group-IV solar cell structures discussed here areextremely useful when integrated into multijunction cells, such as, forexample:

-   -   4J AlGaInP/AlGaInAs/Ge/Ge solar cells;    -   5J AlGaInP/AlGaInAs/GaInAs/Ge/Ge solar cells; and    -   6J AlGaInP/GaInP/AlGaInAs/GaInAs/Ge/Ge solar cells.

In all of these multijunction cells using high-bandgap AlGaInP topsubcells, the emitter may have a lower Al composition (low-Al AlGaInP),or an Al-free composition (GaInP), to improve minority-carrier lifetime,mobility, diffusion length, and blue response and current density of thetop subcell. Though this often results in a lower voltage of theAlGaInP-base top cell, it is often a worthwhile tradeoff because of theincreased short-wavelength quantum efficiency and current produced usinga low-Al or Al-free emitter.

In a conventional 3J GaInP/GaInAs/Ge solar cell there is substantialwasted photogenerated current density in the Ge bottom cell, which canbe used much more effectively with the higher voltage of a 2-junctionGe/Ge cell combination at the bottom of a 4J AlGaInP/AlGaInAs/Ge/Gesolar cell. A 4-junction AlGaInP/AlGaInAs/GaInAs/Ge solar cell has theadvantage of having higher voltage and lower current compared to a 3Jcell, resulting in lower series resistance I²R power losses for currentI and electrical resistance R, but the Ge bottom cell has an evengreater wasted surplus of photogenerated current. As a result, the 4JAlGaInP/AlGaInAs/GaInAs/Ge cell benefits even more from theincorporation of a 2-junction Ge/Ge, or 2× Ge, cell combination as thebottom two subcells in a 5J AlGaInP/AlGaInAs/GaInAs/Ge/Ge solar cell.FIG. 14 shows the cross sectional diagram, calculated subcell andmultijunction cell light I-V curves, and calculated light I-V parametersfor this type of 5J cell. The light I-V parameters are calculationsindicative of expected average production performance of the 5J cell,with an projected average efficiency over 42% under the AM1.5D spectrumat 500 suns (50 W/cm²), significantly higher than the 40% efficiency ofthe best conventional 3-junction cells presently available inproduction. There is still some excess photogenerated current inhigh-performance 2J Ge/Ge subcells in the 5J cell, which can be put tobetter use as discussed below.

FIG. 14 is a cross sectional diagram showing calculated subcell andmultijunction cell light I-V curves, and calculated light I-V parametersfor a 5-junction (5J) AlGaInP/AlGaInAs/GaInAs/Ge/Ge solar cell, using anepitaxial Ge cell 4, and a 2-junction Ge/Ge combination for the bottomtwo cells in the stack. The excess photogenerated current density can beseen in the light I-V curves of Ge subcells 4 and 5 for the 5J cell inFIG. 14. If the bandgap of subcell 3 can be lowered to convert some ofthis photogenerated current at the higher voltage of the GaInAs subcell,and the bandgaps of subcells 1 and 2 can be tuned to maintain currentbalance with this change in subcell, then less current is wasted insubcells 4 and 5 and the 5J cell efficiency increases. One way toachieve a lowering of the effective bandgap of subcell 3 is toincorporate energy wells, or low-bandgap absorber regions (LBARs), intothe intrinsic region of the cell base. Such energy wells may be thinenough that the energy levels of carriers confined in the wells areshifted due to quantum mechanical effects, or may be thick enough thatsuch quantum mechanical energy shifts are negligible. Such energy wellsare typically, but not always, strain-balanced with tensilestrain-compensation layers to counteract the compressive stress of, forinstance, 12%-In GaInAs energy wells in a 1%-In GaInAs cell 3 intrinsicregion. The energy wells in cell 3 may be used in combination with aBragg reflector at the back of cell 3, to reflect back light that wasnot absorbed the energy wells on the first pass, for a second chance ormore at being absorbed. The Bragg reflector can also reflect back thelight generated by radiative recombination in the energy wells, whichcan be quite strong in a high-efficiency cell, giving that lightmultiple chances to be absorbed again by the energy wells, increasingthe subcell 3 current density, steady-state excess carrierconcentration, and subcell voltage.

By lowering the effective bandgap of subcell 3, more current iscollected at the subcell 3 voltage, rather than the relatively lowvoltage of the Ge cells, increasing the efficiency of the 5-junctioncell. FIG. 15 is a cross-sectional diagram showing calculated subcelland multijunction cell light I-V curves, and calculated light I-Vparameters for a 5J cell with an epitaxial Ge cell 4 in a 2J Ge/Ge cellconfiguration at the bottom of the stack, and with energy wells, tolower the effective bandgap of subcell 3, resulting in higher 5J cellefficiency. The light I-V parameters are calculations indicative ofexpected average production performance of this type of 5J cell, with anincreased projected average efficiency over 43% under the AM1.5Dspectrum at 500 suns (50 W/cm²), relative to the upright,lattice-matched 5J cell with no energy wells in cell 3, as depicted inFIG. 14.

Similarly, the entire composition of GaInAs subcell 3 can be shifted tolower bandgap in a metamorphic (MM) solar cell, to make better use ofthe excess photogenerated current in the 2J Ge/Ge bottom cellconfiguration of a lattice-matched 5J cell. The lattice constant ofsubcell 3 is shifted to a large atomic spacing, by virtue of ametamorphic buffer that transitions from the Ge substrate latticeconstant to that of the upper subcells. The bandgaps of subcells 1 and 2are also shifted to lower values at this larger lattice constant, butthis may be compensated for in the top cell by adding more Al.

As before, by lowering the effective bandgap of subcell 3 with thismetamorphic approach, more current is collected at the subcell 3voltage, rather than the relatively low voltage of the Ge cells,increasing the efficiency of the 5-junction cell. FIG. 16 is across-sectional diagram showing calculated subcell and multijunctioncell light I-V curves, and calculated light I-V parameters for a 5J cellwith an epitaxial Ge cell 4 in a 2J Ge/Ge cell configuration at thebottom of the stack, and with metamorphic subcells 1, 2, and 3 (grownwith a relatively modest amount of lattice mismatch with respect to thegrowth substrate) to lower the bandgap of subcell 3, resulting in higher5J cell efficiency. A relatively small amount of lattice mismatch isdesirable, to keep growth costs low for the metamorphic buffer, toreduce wafer bow from strain induced by the lattice mismatch, andthereby reduce yield losses that can stem from wafer bow, and perhapsmost importantly, to minimize dislocations resulting from the latticemismatch, which can cause higher minority-carrier recombination andshunt paths for solar cells. The light I-V parameters are calculationsindicative of expected average production performance of this type of 5Jcell, with an increased projected average efficiency over 43% under theAM1.5D spectrum at 500 suns (50 W/cm²), relative to the upright,lattice-matched 5J cell in FIG. 14.

FIG. 17 shows the plotted results for yet another preferred arrangementcontemplated by the present disclosure with the calculated externalquantum efficiency (EQE), internal quantum efficiency (IQE), and overallabsorptance for a 5-junction cell incorporating an epitaxial,heterojunction Ge cell 4 showing the light response of each subcell. The5-junction cell has an AlGaInP/AlGaInAs/GaInAs/epitaxial Ge/substrate Gecell structure, with a base bandgap combination of2.05/1.68/1.41/0.67/0.67 eV. The top cell (cell 1 or C1) has an AlGaInPemitter bandgap of 1.95 eV in combination with the 2.05 eV base. Thetransparency of the thin epitaxial Ge cell 4 (C4), and its transmissionof some light through to the thick substrate Ge cell 5 (C5) can be seenin the overlapping quantum efficiency curves in FIG. 17.

FIG. 18 shows yet another preferred arrangement contemplated by thepresent disclosure with the measured light I-V characteristics of twofully-integrated prototype 5-junction (5J) cells, incorporating aheterojunction epitaxial Ge cell 4. The 5J cells have anAlGaInP/AlGaInAs/GaInAs/epitaxial Ge/substrate Ge cell structure, withall subcells active, with a 2.05/1.64/1.40/0.67/0.67 eV base bandgapcombination, as shown in the schematic diagram in FIG. 18. Both of the5-junction cells in FIG. 18 interconnect the epitaxial heterojunction Gecell 4 with a substrate Ge cell 5 using a III-V tunnel junction: aGaAs/GaAs tunnel junction as in FIG. 1 for the curve indicated bytriangles in FIG. 18, and an AlGaAs/GaAs tunnel junction as in FIG. 4,for the curve indicated by squares in FIG. 18. The light I-Vmeasurements were made on modified terrestrial quick process (MTQP)solar cells, under an Ioffe solar simulator at a nominal concentrationat greater than 2000 suns (200.0 W/cm²). The 5-junction cells exhibitopen-circuit voltage Voc of approximately 5.2 V under these testconditions. FIG. 18 also shows the light I-V characteristic of a4-junction cell for comparison, which has the same structure as the 5Jcells except that the epitaxial Ge cell 4 and associated tunnel junctionare absent. These control 4J cells have the same short-circuit currentdensity Jsc as the 5J cells, but their Voc is lower at approximately 4.8V. Thus the heterojunction epitaxial Ge cell 4 contributes ˜0.4 V to the5J cell structure, as expected, enabling high-efficiency 5-junctionsolar cells.

Therefore, according to the present disclosure, a preferred solar cellcomprises additional heterostructure layer(s) having higher bandgap(s)than the first photoabsorbing layer, in order: 1) to reduce unwantedphotoabsorption in the additional heterostructure layers(s); 2) tosuppress minority-carrier recombination within and at one or moresurfaces of the additional heterostructure layer; and/or 3) to reduceunwanted dopant or other impurity diffusion from one part of the solarcell to another, particularly since many of these heterostructures mayinclude both group-IV and III-V semiconductors in adjacent layers, andthe elements in these different families of semiconductors act asdopants in the other family of semiconductors, a phenomenon termed hereas cross-column doping, referring to the columns in the periodic tableof elements.

In an alternative, the present disclosure contemplates a solar cell withat least one, first layer composed of a group-IV semiconductor, such asGe, Si, SiGe, SiGeSn, or

CSiGeSn, in which part of the solar cell such as the emitter, base,window, back surface field (BSF) layer, intrinsic layer, or tunneljunction layer comprises a second layer composed of a III-Vsemiconductor or group-IV semiconductor layer with different compositionthan the first group-IV layer, such that a heterostructure is formedbetween the first group-IV layer and the second III-V or group-IV layer.The group-IV layers in the solar cell may be epitaxially-grown, may be aformed from a growth substrate wafer, or may be wafer-bonded layers. Thesolar cell containing at least one group-IV layer may be asingle-junction cell or a subcell in a multijunction solar cell, and theadditional III-V semiconductor or group-IV semiconductor layers formingthe heterostructure may form additional parts of the first solar cell,or may form additional subcells in a multijunction solar cell. Theinvention increases the voltage, current, fill factor, and/or efficiencyof the multijunction or single-junction solar cell.

The heterostructure may allow the formation of the solar cell, as whenthe heterostructure is also a p-n junction formed between the base andemitter of the cell, allowing a multijunction or single-junction solarcell to be formed with greater voltage, current, fill factor, and/orefficiency. The heterostructure may also be an isotype heterostructure,such as between the emitter and window, between the window and n-typeside of a tunnel junction, between the base and BSF, or between the BSFand p-type side of a tunnel junction, allowing a multijunction or singlejunction solar cell to be formed with greater voltage, current, fillfactor, and/or efficiency. The heterostructure may also comprise atunnel junction, such as when the heterostructure is a p-n junction thatforms a tunnel junction, allowing a multijunction or single-junctionsolar cell to be formed with greater voltage, current, fill factor,and/or efficiency.

The invention may be used in a lattice-matched, upright-grownmultijunction solar cell, with significant advantages in growth cost,yield, wafer bowing, and processing cost compared to highly-latticemismatched metamorphic cells or cells with an inverted-growth structure.However, the present invention may be used to advantage in multijunctioncells with layers having a relatively small amount of lattice mismatch,in either metamorphic layers or pseudomorphically-strained layers suchas low-bandgap absorber layers, or in pseudomorphically-strained window,emitter, base, BSF, intrinsic layers or tunnel junction layers. Also,the present invention may also be used to advantage in inverted-growthstructures that are lattice-matched or have a relatively small amount oflattice mismatch.

According to the present disclosure, preferred variations include agroup-IV solar cell that is optically thin in order to leak asubstantial amount of light to an additional subcell beneath, e.g., anepitaxially-grown Ge, SiGe, or SiGeSn subcell on top of a Ge substratesubcell in a multijunction solar cell, in which most or all of thephotoabsorbing group-IV layer in the group-IV solar cell is doped n-typeby the growth of subcells on top, and/or by III-V semiconductor layerson which the group-IV solar cell is grown, such that the p-n junction isformed at the back of the main photoabsorbing group-IV layer.

Further alternatives include a group-IV solar cell in which the mainphotoabsorbing layer is an epitaxially-grown Ge, SiGe, or SiGeSn n-typeemitter layer, doped n-type by the nucleation and growth of III-Vsemiconductors on top of it, from which photogenerated minority holesare collected at a p-n junction at the back of this n-type Ge, SiGe, orSiGeSn emitter layer.

According to further variations, the present disclosure contemplates ann-type Ge, SiGe, SiGeSn emitter solar cell, such as those describedherein, with thicknesses in the range of from about 0.01 to about 10microns, and preferably in the range of from about 0.1 to about 2microns, and still more preferably in the range of from about 0.3 toabout 1 micron.

As shown in FIG. 1, examples of the n-type Ge, SiGe, SiGeSn emittersolar cell described above are further contemplated by the disclosure,where 1) the p-n junction at the back of the emitter layer is formed bya p-type layer, which stops the n-type doping that diffuses in from thefront of the cell, since the group-V elements that cause n-type dopingin group-IV semiconductors are not dopants in semiconductors; 2) wherethe p-n junction at the back of the emitter layer is a heterojunctionbetween the n-type group-IV emitter and a p-type III-V semiconductorbase or BSF layer; and 3) where the p-n junction at the back of theemitter layer is a heterojunction between the n-type group-IV emitterand a p-type GaAs base or BSF layer.

Further contemplated variations of the present disclosure include anupper group-IV solar cell that is optically thin, that transmits roughlyhalf of the light energy incident upon it to a second, lower group-IVsolar cell positioned beneath the first, upper group-IV cell, such thatit is in optical series with the first cell. The upper group-IV cell andthe lower group-IV cell may be connected in electrical series by atunnel junction, which it is desirable to make as transparent aspossible to the wavelengths used by the second, lower cell. Calculatedquantum efficiencies of a thin, upper, epitaxial Ge subcell 4 in a 5Jcell, and of an optically thick, lower Ge substrate subcell 5 in a 5Jcell, are plotted as a function of wavelength for various upper subcellthicknesses in FIG. 2, using Ge absorption coefficients. The resultingcurrent balance for various upper subcell thicknesses can be seen inFIG. 2. The current densities of both the upper and lower group-IVsubcells, and the ratio of their currents, called the J-ratio, isplotted as a function of thickness in FIG. 3.

Presently contemplated solar cell further include those where theadditional heterostructure layer(s) have higher bandgap(s) than thefirst photoabsorbing layer, in order:

1) to reduce unwanted photoabsorption in the additional heterostructurelayers(s);

2) to suppress minority-carrier recombination within and at one or moresurfaces of the additional heterostructure layer; and/or

3) to reduce unwanted dopant or other impurity diffusion from one partof the solar cell to another, particularly since many of theseheterostructures may include both group-IV and III-V semiconductors inadjacent layers, and the elements in these different families ofsemiconductors act as dopants in the other family of semiconductors, aphenomenon termed here as cross-column doping, referring to the columnsin the periodic table of elements.

Any of the group-IV subcell combinations discussed earlier, such as aSiGeSn cell 4/Ge cell 5 combination, may be used in place of the2-junction Ge/Ge bottom cells in 4-junction and 5-junction cells. Inaddition, any of the group-IV subcell heterostructures may be used inthe bottom group-IV cells of 3-junction, 4-junction and 5-junctioncells. Similar arguments apply to cells with 6-junctions or more.

The group-IV solar cell layers and solar structures contemplated here inwhich the group-IV semiconductors are employed are designed to make usecertain natural electronic band structure features and semiconductorparameters of group-IV materials.

The optically-thin upper Ge, SiGe, or SiGeSn subcells can have a smallphysical thickness due to the high absorption coefficient of thesematerials relative to many indirect gap semiconductors, because of adirect gap transition that exists close to and above the indirect gap inenergy for many compositions of these group-IV semiconductors. Forexample, Ge absorbs strongly for photon energies above the ˜0.8-eVdirect gap in this material, allowing relatively small thicknesses onthe order of 0.5 microns to be sufficient for a Ge upper subcell forcurrent matching, with the attendant low growth times and low cost ofepitaxial growth, even though Ge is an indirect semiconductor with anindirect gap slightly lower at 0.67 eV. When charge carriers thermalizeto the lower indirect gap, the requirement for an electron-holerecombination event to involve 3 bodies in order to take place—theelectron, the hole, and also a phonon—resulting in a very long minoritycarrier lifetime compared to direct gap semiconductors. Coupled with thevery high carrier mobilities in Ge, these long lifetimes result in verylong minority-carrier diffusion lengths, making these group-IV solarcells highly tolerant to defects, impurities, and other imperfectionsthat tend to increase undesirable Shockley-Read-Hall (SRH)recombination. Thus a thin solar cell with low-cost growth, combininghigh absorption coefficient, and high tolerance to defects and non-idealrecombination mechanisms can be achieved with Ge, SiGe, SiGeSn, andother group-IV solar cells, and it is contemplated that the best solarcells will adjust the compositions of the group-IV solar cells tomaximize these effects for a given solar cell application.

In addition, it is contemplated that the band structure of group-IVlayers in the solar cell may be designed, controlled, and manipulated bycompressive or tensile strain in the layers, and by selection ofgroup-IV composition, to achieve desirable optical and electricalproperties in the solar cell. For example, compressive or tensile strainfor certain group-IV semiconductor compositions may increase theirbandgap, making them more transmissive to incident light which isdesirable for layers such as tunnel junction layers, window layers, andBSF layers in which minority-carrier collection probability may beimpaired, and can be desirable in emitter and base layers to adjustsubcell bandgap and current collection, and increase subcell voltage.Compressive and tensile strain, and group-IV composition may also beused to adjust whether the semiconductor has an indirect gap or a directgap as the lowest energy transition, may be used to adjust the energydifference between indirect and direct bandgaps, and may be used toadjust effective densities of states in the conduction and valancebands, affecting photoabsorption in the layers, as well as the ease atwhich the semiconductors may be doped degenerately with the Fermi levelresiding in the conduction or valance band of allowed energies,impacting the ease and doping level at which tunnel junctions may beconstructed in the group-IV semiconductor material.

Structures with selective wavelength reflection and anti-reflection (AR)structures may be combined with group-IV solar cells, and theirreflectance properties may be tuned to increase the performance of thegroup-IV solar cell, and/or a multijunction solar cell in which it isused, for example: Bragg reflector structures using semiconductormaterials with different refractive indices, may be incorporated behinda group-IV solar cell to increase the optical path length and currentdensity of that cell, such as in an epitaxial cell 4 or substrate cell 5in an upright 5-junction solar cell, epitaxial cell 4 and/or epitaxialcell 5 in an inverted 5-junction solar cell, epitaxial cell 4 and/orepitaxial cell 5 and/or substrate cell 6 in an upright 6-junction solarcell, etc.; back surface reflector structures using a metal/dielectric,metal/semiconductor, and/or semiconductor/dielectric structure, inaddition to all-semiconductor structures may be incorporated with agroup-IV cell to increase its current density, such as the substratecell 5 of an upright 5-junction cell, the epitaxial cell 5 of aninverted 5-junction solar cell, or the bottom cell of an upright orinverted multijunction solar cell with 2, 3, 4, 5, 6 or more junctions.Multilayer AR coatings on the front or the back of the solar cell may betuned with regard to their reflective properties as a function ofwavelength, in order to increase the performance of a multijunction cellincorporating a group-IV solar cell.

Although most examples here have discussed an n-on-p solar cellconfiguration, in which light first strikes the n-type side of the solarcell p-n junction, similar principles can be adapted to apply to, andthe present disclosure contemplates, p-on-n solar cell configurations aswell.

In addition, although most examples here have discussed structures thatmake use of or can function with cross-column doping between group-IVsemiconductors and III-V semiconductors, similar principles can also beadapted to, and the present disclosure contemplates, devicesincorporating group-IV, III-V, II-VI, I-III-VI, III-IV-VI, and otherfamilies of semiconductors, and the cross-column doping that can takeplace between them.

Further, although most examples shown here discuss group-IV solar cellswith group-IV semiconductor layers formed from column-IV elements alone,other chemical compounds are contemplated, such as those that includeone or more group-IV elements in combination with one or more group-IIIelements, e.g., boron in combination with silicon, BSi; AlSi; or GaGe,or that include one or more group-IV elements in combination with one ormore group-V elements, e.g., phosphorus in combination with silicon,SiP; SiN; or GeAs. Such compounds may be useful for highly doped layerssuch as tunnel junction layers, or may be used for other layers of thesolar cell such as contact, window, emitter, base, and BSF regions.

Although most examples shown here may use III-V semiconductors for thecontact layers (also known as cap layers), group-IV semiconductors suchas Ge, Si, SiGe, SiGeSn, and CSiGeSn, may be used as the contact or caplayers. These layers have relatively low bandgaps compared to many III-Vsemiconductors, helping to lower contact resistance between thesemiconductor contact layer and a metal contact, and group-IV materialscan be grown lattice-matched to a variety of III-V compounds,maintaining high crystal defect density. Since in an upright growthconfiguration the front contact or cap layer is the last layer grown,even if the contact material, e.g., a group-IV contact material, has ahigh lattice mismatch to the III-V layers it is grown on (such as alower layer of the contact or cap structure formed from III-Vsemiconductors), the structure can be highly tolerant of the highlattice mismatch, since no further semiconductor layers will be grown ontop of the last contact layer, and hence the dislocation density in thelast contact layer can be quite high without harming active layers inthe device for which long minority-carrier lifetime is important. It isalso considered in this disclosure that group-IV layers grown as thelast layer or layers on the top of the multijunction solar cell may beused to construct bypass diodes for the purpose of protecting themultijunction solar cell or other multijunction cells in the circuit, orblocking diodes or other types or electronic and optoelectronic devicesthat are grown monolithically with the multijunction cell itself, foruse in the solar cell circuit.

Still further, although most examples here have discussed upright,lattice-matched multijunction cells, the subcells in the multijunctioncell, including the group-IV subcells described in this disclosure, maybe grown inverted, such as, for example, with the sunward surface of thecell grown first, and in a multijunction cell with the higher bandgapsubcells grown first, and/or may be grown as metamorphic cells,lattice-mismatched with respect to the growth substrate or to thesurface on which they are grown. A potential advantage of invertedgrowth may be that the lower group-IV cells and associated tunneljunctions will experience less degradation from the thermal budget ofthe other subcells in an inverted configuration, since the lowergroup-IV cells will be grown after the upper subcells in an invertedconfiguration. A potential advantage of metamorphic orlattice-mismatched growth is the larger range of semiconductormaterials, compositions, and bandgaps this approach allows. For example,metamorphic SiGe subcells may be grown on Si substrates, metamorphicSiGe subcells may be grown on Ge substrates, metamorphic CSiGeSnsubcells may be grown on Si or Ge substrates, lattice-mismatched ormetamorphic Ge subcells may be grown as the bottommost cell (cell 4) ina 4-junction inverted metamorphic cell design, replacing the ˜0.7-eVmetamorphic GaInAs cell usually used in that position, thus reducing thegrowth time required for the second metamorphic buffer normally requiredin usual 4-junction inverted metamorphic cell design, etc.

While most examples here have discussed a monolithic, epitaxially-grownmultijunction cell structure that may be formed in 1, 2, 3, or moreepitaxial growth runs on the same growth substrate, the presentdisclosure contemplates that the subcells in the multijunction cell,such as, for example, the epitaxial group-IV and/or substrate group-IVcell or cells, also may be integrated into the multijunction structureusing wafer bonding, or semiconductor bonding technology (SBT), or usinga combination of monolithic growth and SBT. Examples include the upper3-junction AlGaInP/AlGaInAs/GaInAs cell combination semiconductor bondedor bonded with a metal/dielectric interface to lower group-IV cells,such as, for example, a 1-junction Ge, SiGe, or SiGeSn cell, a2-junction Ge/Ge, SiGe/Ge, or SiGeSn/Ge cell, or 3-junctionSiGeSn/SiGeSn/Ge group-IV cell, to form 4, 5, or 6-junction SBT cellsthat incorporate one or more group-IV subcells.

Still further, while most examples herein have discussed group-IV solarcells with heterostructures formed between solar cell layers such asbetween emitter and base, or between base and BSF, etc., the presentdisclosure contemplates that heterostructures may also be formed withinthese solar cell layers, with lower bandgap, higher bandgap, or the samebandgap as the surrounding material in the solar cell layer. Forexample, energy wells may be placed in the main absorber layer of agroup-IV solar cell that may be an emitter, base, or other layer of asolar cell, to lower the effective bandgap of the group-IV absorberlayer, such as, for example, strained or unstrained Ge, SiGe, or SiGeSnenergy wells and/or strain-balance layers in a group-IV absorber layergrown on a silicon substrate, such as, for example, metamorphic SiGe orpseudomorphic SiGeSn absorber layers, GeSn or SiGeSn energy wells in aGe absorber to lower the effective bandgap of cell 5 in a 5-junctioncell below that of Ge, GeSn energy wells in combination with SiGestrain-balance layers, and/or unstrained Ge energy wells in SiGeSn solarcell absorber layers.

While most examples here have discussed forming the group-IV solar celllayers using epitaxial growth, such as, for example: chemical vapordeposition (CVD), ultra-high-vacuum chemical vapor deposition (UHVCVD),metal-organic vapor-phase epitaxy (MOVPE), molecular beam epitaxy (MBE),liquid phase epitaxy (LPE), and others; or by dopant diffusion into agroup-IV substrate, the present disclosure also contemplates that thegroup-IV solar cell layers may also be formed by other methods. Theseinclude, for example, physical vapor deposition (PVD) with or withoutsubsequent annealing and/or recrystallization, and ion implantation ofdopants and/or main components of the group-IV semiconductor into astarting group-IV substrate, such as, for example: implantation of Siinto a Ge substrate to form a SiGe solar cell layer; Si and Sn into a Gesubstrate to form a SiGeSn cell layer; carbon (C) into a Ge substrate toform a CGe layer; Ge into a silicon (Si) substrate to form a SiGe solarcell layer; Ge and Sn into a Si substrate to form a SiGeSn cell layer;n-type dopants into a p-type group-IV substrate, or otherepitaxially-grown, diffused, or ion-implanted layer, to form an n-typelayer, and/or p-type dopants into an n-type group-IV substrate, or otherepitaxially-grown, diffused, or ion-implanted layer, to form a p-typelayer.

In addition, although most examples here have discussed the formation ofgroup-IV solar cells with the goal of incorporating them into amultijunction solar cell in combination with another group-IV solar cellto form a 2-junction group-IV solar cell pair within a largermultijunction stack which includes additional III-V subcells, thepresent disclosure also contemplates applying the principles disclosedherein to: single-junction group-IV solar cells; to group-IV solar cellcombinations incorporating 3 or more group-IV solar cells in amultijunction solar cell stack; and/or to multijunction cells thatinclude subcells formed from group-IV, III-V, II-VI, III-IV-VI, andadditional families of semiconductors.

Although most examples here have discussed solar cells, the disclosurefurther contemplates usefulness with other types of optoelectronicdevices employing photovoltaic cells and systems. Indeed, any requiredneed for sustainable energy storage and deployment would find use andbenefit from the present disclosure. For example, due to the increasedefficiency of the photovoltaic cell systems of the present disclosure,manned or unmanned vehicular operation in a terrestrial and/ornon-terrestrial setting are made possible. Contemplated vehicles includemanned and unmanned aircraft, spacecraft, terrestrial, non-terrestrialand surface and sub-surface water-borne vehicles.

While the preferred variations and alternatives of the presentdisclosure have been illustrated and described, it will be appreciatedthat various changes and substitutions can be made therein withoutdeparting from the spirit and scope of the disclosure. Accordingly, thescope of the disclosure should only be limited by the accompanyingclaims and equivalents thereof.

We claim:
 1. A photovoltaic cell comprising: at least two group-IVsubcells within a multijunction cell, wherein at least one upper subcellis positioned closer to a primary light source than at least one lowersubcell, and wherein the at least one upper subcell has a higher director indirect energy bandgap that the lower subcell.
 2. The photovoltaiccell of claim 1, wherein the compositions for the upper and lowersubcells are selected from the group consisting of: Ge, Si, SiGe, CGe,GeSn, SiGeSn, and CSiGeSn.
 3. The photovoltaic cell of claim 1, whereinat least one upper subcell comprises SiGeSn, and at least one lowersubcell comprises Ge.
 4. The photovoltaic cell of claim 1, wherein atleast one of the two or more group-IV solar cells further comprise: afirst layer comprising a solar cell emitter layer formed from a group-IVlayer, and a second layer comprising tunnel junctions formed from one ormore III-V semiconductor layers, wherein the one or more tunnel junctionsemiconductor layers are selected from the group consisting of: GaAs,AlGaAs, GaInAs, AlGaInAs, InP, GaP, GaInP, GaInPAs, AlInP, AlGaInP,InAs, InPAs, AlInAs, AlAs, GaSb, GaAsSb, InSb, GaInAsSb, GaInNAs,GaInNAsSb, GaN, AlN, InN, GaInN, AlGaN, AlInN, AlGaInN, and combinationsthereof.
 5. The photovoltaic cell of claim 1, wherein at least one ofthe two or more group-IV solar cells further comprise: a first layercomprising a solar cell emitter layer formed from one or more group-IVsemiconductor layers, and a second layer comprising tunnel junctionsformed from one or more group III-V semiconductor layers, wherein thetunnel junctions are selected from the group consisting of:p+GaAs/n+GaAs tunnel junctions; p+AlGaAs/n+GaAs tunnel junctions;p+GaAs/n+AlGaAs tunnel junctions; p+AlGaAs/n+AlGaAs tunnel junctions;p+AlGaAs/n+GaInP tunnel junctions; p+AlGaAs/n+AlGaInP tunnel junctions;and p+AlGaInP/n+AlGaInP tunnel junctions.
 6. The photovoltaic cell ofclaim 1, wherein at least one of the two or more group-IV solar cellsfurther comprise: a first layer comprising a solar cell emitter layerformed from at least one group-IV semiconductor material, and a secondlayer comprising tunnel junctions formed from at least one group-IVsemiconductor layers, wherein the one or more tunnel junctionsemiconductor layers is selected from the group consisting of: Ge, Si,SiGe, CGe, GeSn, SiGeSn, and CSiGeSn.
 7. The photovoltaic cell of claim1, wherein at least one of the two or more group-IV solar cells furthercomprises: a first layer comprising a solar cell emitter layer formedfrom at least one group-IV semiconductor materials, and a second layercomprising tunnel junctions formed from at least one group-IVsemiconductor layers, which are selected from the group consisting of:strained or unstrained p+Ge/n+Ge tunnel junctions; strained orunstrained p+SiGe/n+SiGe tunnel junctions; strained or unstrainedp+CGe/n+CGe tunnel junctions; strained or unstrained p+SiGeSn/n+SiGeSntunnel junctions; strained or unstrained p+CSiGeSn/n+CSiGeSn tunneljunctions; strained p+SiGe/n+Ge tunnel junctions; p+Ge/strained n+SiGetunnel junctions; strained p+CGe/n+Ge tunnel junctions; p+Ge/strainedn+CGe tunnel junctions; p+SiGeSn/n+Ge tunnel junctions; p+Ge/n+SiGeSntunnel junctions; p+CSiGeSn/n+Ge tunnel junctions; and p+Ge/n+CSiGeSntunnel junctions.
 8. The photovoltaic cell of claim 1, wherein the twoor more group-IV solar cells are subcells within a multijunctionphotovoltaic cell, and the multijunction photovoltaic cell is selectedfrom the group consisting of: a 2-junction cell, 3-junction cell,4-junction cell, 5-junction cell, 6-junction cell, and multijunctioncells with 7 or more collecting junctions or subcells, whereinn-junction cells where n is an integer greater than or equal to 2 andwherein the multijunction cell further comprises a group-IV material toprovide a selected bandgap combination among the subcells of themultijunction cell.
 9. The photovoltaic cell of claim 1, wherein atleast one of the two or more group-IV solar cells is a subcell within amultijunction photovoltaic cell that comprises at least one group-IVsemiconductor layer that is epitaxially grown in a deposition apparatus,and wherein the multijunction cell comprises at least one additionalsubcell comprising III-V semiconductor layers.
 10. The photovoltaic cellof claim 1, wherein at least one of the two or more group-IV solar cellscomprises an emitter layer and a base layer, wherein the emitter layeris positioned closer to the primary light source for the solar cellduring solar cell operation than to the base layer.
 11. The photovoltaiccell of claim 1, wherein at least one of the two or more group-IV solarcells comprises an emitter layer and a base layer, wherein the baselayer is positioned closer to the primary light source for the solarcell during solar cell operation than to the emitter layer.
 12. Thephotovoltaic cell of claim 1, wherein at least one of the two or moregroup-IV solar cells comprises an emitter layer and a base layer,wherein the emitter layer is positioned at approximately the samedistance from the primary light source for the solar cell during solarcell operation as from the base layer.
 13. The photovoltaic cell ofclaim 1, wherein at least one of the two or more group-IV solar cellscomprises an emitter layer and a base layer, wherein the emitter layeris formed after the base layer of the solar cell.
 14. The photovoltaiccell of claim 1, wherein at least one of the two or more group-IV solarcells comprises an emitter layer and a base layer, wherein the baselayer is formed after the emitter layer of the solar cell.
 15. Thephotovoltaic cell of claim 1, wherein at least one of the two or moregroup-IV solar cells comprises an emitter layer and a base layer,wherein the emitter layer is formed at approximately the same time asthe base layer of the solar cell.
 16. The photovoltaic cell of claim 1,wherein at least one of the two or more group-IV solar cells comprisesan emitter layer and a base layer, wherein the emitter layer comprises alower energy bandgap than the base layer of the solar cell.
 17. Thephotovoltaic cell of claim 1, wherein at least one of the two or moregroup-IV solar cells comprises an emitter layer and a base layer,wherein the emitter layer comprises a higher energy bandgap than thebase layer of the solar cell.
 18. The photovoltaic cell of claim 1,wherein at least one of the two or more group-IV solar cells comprisesan emitter layer and a base layer, wherein the emitter layer hasapproximately the same energy bandgap as the base layer of the solarcell.
 19. A vehicle comprising the photovoltaic cell of claim
 1. 20. Anenergy storage system comprising the photovoltaic cell of claim
 1. 21.An energy generation system comprising the photovoltaic cell of claim 1.22. A method for energy generation comprising the step of: providing aphotovoltaic cell comprising at least two group-IV subcells within amultijunction cell, wherein at least one upper subcell is positionedcloser to a primary light source than at least one lower subcell, andwherein the at least one upper subcell has a higher direct or indirectenergy bandgap that the lower subcell.
 23. The method of claim 22,wherein the compositions for the upper and lower subcells are selectedfrom the group consisting of: Ge, Si, SiGe, CGe, GeSn, SiGeSn, andCSiGeSn.
 24. The method of claim 22, wherein at least one upper subcellcomprises SiGeSn, and at least one lower subcell comprises Ge.